The ability to smell and, in part, the ability to taste is regulated by the olfactory nerve system. The olfactory nerve system is complex and interconnected with several systems in the brain. Olfactory receptors located in the nose are specialized bipolar neurons with cilia protruding into the mucous covering the epithelium. The axons of the bipolar neurons are packed into bundles that form connections in the olfactory bulb in the brain. The olfactory bulbs contain a rich supply of neurotransmitters and neuromodulators. Neuromodulators include thyrotropin releasing hormone, substance P, enkephalin, dopamine, glutamate, and aspartate. The neurotransmitters include serotonin, acetylcholine and noradrenaline which are delivered to the bulbs from cell bodies in other brain regions and are formed within the bulbs in the terminal projections only. Central olfactory projections from the bulb interconnect the bulb to other areas of the brain, including the hippocampus, the hypothalamus, and the pyriform lobe.
There is an anatomical and biochemical connection between the olfactory system and the limbic system in the brain. The limbic system includes the hippocampus and amygdala region, and is known as the emotional center of the brain. The limbic regions have many synaptic contacts with olfactory bulbs. Many of the limbic structures and the olfactory bulbs are reciprocally interconnected in loop pathways that may be involved in the regulation of brain emotional output.
There are several known disorders of taste and smell which affect the function of the olfactory system and which present major problems for the patient. Chemosensory dysfunctions are usually described by the following terms: ageusia (absence of taste), hypogeusia, (diminished sensitivity of taste), dysgeusia (distortion of normal taste), anosmia (absence of smell), hyposmia (diminished sense of smell), and dysosmia (distortion of normal smell). These disorders cause modification of food choices and dietary habits, alter digestion, and the ability to detect noxious gases and poisons. Overall, chemosensory disorders are chronic problems that can reduce enjoyment and quality of life.
It is also known that neurological disorders involving damage to the brain can also include a chemosensory dysfunction. For example, patients suffering from Alzheimer's disease show a marked impairment in smell identification which may be associated with senile plaques, neurofibrillary tangles, and reduced cholinergic activity in the olfactory bulb.